(1) Field of the Invention
The present invention relates to means for influencing the flow of air over the outside surfaces of rotorcraft. The present invention relates more particularly to a fairing conventionally arranged at the top of a rotorcraft being located under the rotary wing of a main rotor of substantially vertical axis and above a cover covering a power plant of the rotorcraft.
(2) Description of Related Art
A rotorcraft is a rotary wing aircraft having one or more rotors, including at least a main rotor of substantially vertical axis arranged at the top of the rotorcraft. The main rotor typically provides the rotorcraft at least with lift, and in the specific circumstance of a helicopter, possibly also with propulsion and/or guidance along its movement axes.
A rotorcraft may also have at least one auxiliary rotor of substantially horizontal axis. Conventionally, such an auxiliary rotor may be installed at the end of a tail boom in order to control the yaw attitude of the rotorcraft. Such an auxiliary rotor can also be used to provide a rotorcraft with a propulsive propeller for moving the rotorcraft in translation, as for a helicopter having high-speed propulsion in translation.
The rotor(s) of a rotorcraft is/are conventionally driven in rotation by a power plant including at least one fuel-burning engine, in particular a turboshaft engine. The power plant is typically located in the top portion of the rotorcraft beneath the wing of the main rotor. A cover is arranged around the power plant in order to direct the flow of air towards the rear of the rotorcraft and enhance the aerodynamic characteristics of the rotorcraft.
Such a cover is commonly raised by a fairing that covers a mechanism for operating blades of the rotary wing of the main rotor in compliance with flight commands generated by a pilot of the rotorcraft. Such a fairing is commonly referred to as a “hub fairing” or “pylon”.
For example, such a mechanism for controlling the blades of the rotary wing of the main rotor is commonly formed by a pair of “swashplates” movably mounted on a mast carrying the rotary wing. The swashplates are operated by a pilot of the rotorcraft using various control lines in order to cause the pitch of the blades of the rotary wing of the main rotor to vary collectively and/or cyclically so as to modify the attitude of the rotorcraft.
The cover and the fairing on top of it extend mainly in a longitudinally-extending direction of the rotorcraft, conventionally considered as going between the front and the rear of the rotorcraft when on the ground.
Naturally, concepts such as “top”, “high portion”, “below”, “bottom”, or “raising”, for example are commonly understood relative to the vertical orientation of the rotorcraft when on the ground. Likewise, concepts such as “vertical plane” and “horizontal plane” of the rotorcraft are commonly understood respectively relative to the vertical and horizontal general orientations in which the rotorcraft extends when on the ground.
In this context, it is appropriate to arrange the fairing so as to reduce its aerodynamic drag as much as possible and so as to limit the wake that it generates. The arrangement of the fairing must in particular encourage the stream of air to remain attached along its surface as well as along the surface of the cover provided around the power plant underlying the fairing.
In addition, it is also appropriate to arrange the fairing in order to minimize its wake, in order to stabilize its wake as a function of the flight situation, and in order to avoid giving rise to structural excitation of the outside walls of the rotorcraft, in particular the walls extending towards the rear of the rotorcraft such as the walls of the tail boom and the walls forming stabilizing aerodynamic surfaces.
As described in documents U.S. Pat. No. 3,130,942 and US 2011/0036954, a common solution consists in shaping the profile of the fairing in question to have the shape of a drop of water in a horizontal plane of the rotorcraft. It has nevertheless been found that such a solution is not fully satisfactory for optimizing stabilization of the rotorcraft in the event of a side wind and/or of the rotorcraft sideslipping in yaw, in particular for light rotorcraft where it is desired to enhance their maneuverability in flight.
More particularly, among the constraints on how the fairing should be arranged, the flying quality of the rotorcraft must be preserved. In particular, it is necessary to avoid the flow of air along the fairing giving rise to instabilities in maintaining a flight path and/or to disturbances in yaw attitude control of the rotorcraft.
It is found that when the flow of air around the rotorcraft includes a significant sideways vector component, the regularity of the flow of said turbulence towards the rear of the fairing is disturbed, and consequently the stability of the attitude of the rotorcraft in yaw is affected thereby. More particularly, the turbulence generated at the rear end of the fairing tends to become chaotic with frequency spreading and also tends to move rearwards, potentially impacting against the rear aerodynamic portions.
Such instability in the flow of the turbulence, which is made worse in the event of a side wind and/or of the rotorcraft sideslipping in yaw, has the effect of producing unwelcome vibration that may harm the mechanical strength of the rotorcraft and that can degrade passenger comfort. Furthermore, such a situation tends to destabilize the yaw behavior of the rotorcraft.
In order to mitigate those drawbacks, it is known to truncate the waterdrop-shaped profile of the fairing transversely at its rear end. The effect of this is to improve the regularity of the flow of the turbulence in the event of a side wind and/or of sideslipping in yaw. Nevertheless, arranging the trailing edge of the fairing in that way tends to increase its aerodynamic drag, which is undesirable.
In summary, it is found that various specific arrangements for shaping the fairing in order to improve the flow of air for certain flight situations are unsuitable for other flight situations. As a result, it is necessary to find compromises between various arrangements for the fairing in order to obtain the best possible flow of air that is satisfactory regardless of the flight situation of the rotorcraft.
It is also known for the rear wall of the fairing that is formed by truncating its waterdrop-shaped profile to be arranged as a plane that slopes relative to the horizontal plane of the rotorcraft. Such provisions seek to enhance the guidance of the flow of air along the fairing towards the bottom of the rotorcraft, it being understood that the rotation of the main rotor itself generates a powerful flow of air directed towards the bottom of the rotorcraft together with complex aerodynamic phenomena that disturb the stability of the behavior of the rotorcraft in flight.
Consequently, it is desirable for the overall arrangement and for specific features of the fairing concerning the flow of air along its surfaces to take account also of the wash generated by the main rotor.
However minor they might be, the various features applied to the fairing for the purpose of influencing the flow of air along its surfaces, and consequently along members arranged to the rear of the rotorcraft, interfere in combination with one another on the overall conditions of such a flow and of its effects.
Consequently, choices need to be made concerning using the various fairing features singly or jointly in order to obtain a flow of air towards the rear of the rotorcraft that is judged to be satisfactory at best.
It is also desirable for the arrangement of the fairing to be usable without major modification, regardless of the general configuration of the rotorcraft, in order to avoid preparing a specific shape for the fairing that depends on the general structure of the rotorcraft, which structure varies among various families of rotorcraft.
As a result, there is a continuing search for a way of organizing a fairing so as to enhance a flow of air along its outside surface and obtain results that are satisfactory at best concerning the various constraints and requirements mentioned above.